[PECOFF] Writer: Remove NULL padding at the end of each section.

If section size is not multiple of 512, the writer added NULL bytes at the end
of it to make it so. That is not required by the PE/COFF spec, and the MSVC's
linker does not do that too. So we don't need to do that, too.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@197002 91177308-0d34-0410-b5e6-96231b3b80d8